1. Field of Application
The present invention relates to a power steering apparatus for a vehicle, which suppresses vehicle vibration that may occur due to lateral forces acting on tires of the vehicle while the steering direction of the corresponding vehicle wheels is being changed.
2. Description of Prior Art
With a typical power steering apparatus, a signal (referred to in the following as the steering signal) is generated in accordance with the degree to which the steering wheel is being actuated by the vehicle driver, e.g., based on sensing the amount and direction of torque that is being applied to the steering column by the driver. The steering signal is produced from a power steering ECU (Electronic Control Unit), and an EPS (Electrical Power Steering) motor is driven based on that signal, to apply a supplementary amount of torque to a steering mechanism linked to the front wheels, such that the angle of the front wheels is adjusted appropriately. Such a system is described in Japanese Patent No. 2568817 (corresponding to U.S. Pat. No. 4,664,211) for example.
With such a system, when the steering angle of the front wheels is changed so that the vehicle begins to attempt to turn, lateral forces are thereby applied to each of the tires of the front wheels, causing twisting of the rubber of the tires to occur, which may produce vibration. This vibration is transmitted through the steering mechanism to the shaft of the EPS motor, and can result in angular shaking of the rotor of the EPS motor. This can result in electric power being generated by the EPS motor, with a generated voltage being produced which is of inverse direction to the drive voltage of the motor. This phenomenon occurs repetitively, causing pulsation in the effective drive current of the EPS motor, with resulting vibration of the EPS motor, which causes noise to be produced by the EPS motor. In addition, the vibration may result in resonant vibration of the vehicle body and the steering wheel, causing sensations of discomfort to the driver. Furthermore, this phenomenon results in instability of the reaction forces between the tires and the road surface, so that there is a lowering of stability of vehicle motion.
As an attempt to overcome this problem, a system has been proposed whereby the steering shaft is divided into a first steering column that is connected to the steering wheel and a second steering column that is connected to the lower end of the first steering column via a flexible member. That is to say, a mechanical arrangement is used to attempt to reduce the amount of vibration of the steering wheel. This system is described for example in Japanese Patent Laid-open No. 2001-151123.
However with the latter prior art method, there is no reduction achieved in vibration of the EPS motor. Hence, there will be no lowering of the vibration noise that is produced by the EPS motor, so that there will be no reduction of the degree of driver discomfort which results from the transfer of vibration to the body of the vehicle from the EPS motor.